This invention is concerned with the purification of 2,2-bis(4-hydroxyphenyl) propane (herein identified as "bisphenol-A" or "BPA"). More particularly, this invention is directed to a method of recovering bisphenol-A in a purified state from crude bisphenol-A by treatment with an aqueous alkaline solution.
Crude bisphenol-A is the isolated product of commercial processes for preparing bisphenol-A. It is a mixture of bisphenol-A and impurities derived from a BPA synthesis reaction. An example of a BPA synthesis reaction is the acid-catalyzed condensation of phenol and acetone, where phenol and acetone react in the presence of an acidic material, such as sulfuric acid, hydrochloric acid, cation exchange resin, etc.
The crude bisphenol-A produced contains undesirable impurities such as phenol, 2-(4-hydroxyphenyl)-2-(2-hydroxyphenyl) propane having the formula ##STR1## a trishydroxyphenyl compound of the formula ##STR2## small amounts of other impurities such as the two compounds having formulas ##STR3## and some impurities which discolor the crude bisphenol-A with unknown structure (herein identified as color bodies).
Since bisphenol-A is used in making polycarbonate resins by reaction of the former with either phosgene or diphenyl carbonate, or for making epoxy resins, both resins being used extensively in commercial applications involving molding, casting and sheet forming processes, it is highly important that the monomeric bisphenol-A used to make such resins be as pure as possible in order to avoid adverse effects on the properties of the polymers thus obtained.
The preparation of bisphenol-A by the acid catalyzed reaction of phenol and acetone is usually carried out in excess phenol (2 or more moles per mole of acetone). This reaction mixture is either subjected to a series of distillation steps to remove substantially all phenol, acetone and water or the acetone and water are removed and the bisphenol-A product is crystallized in the presence of phenol, followed by stripping of the phenol. Both procedures provide crude bisphenol-A which is the starting point for making bisphenol-A of high purity. Distillation or stripping of the phenol from the reaction mixture can be carried out only to a limited extent on account of the thermal instability of the bisphenol-A product.
A conventional method of recovering pure bisphenol-A product from crude bisphenol-A involves crystallization in the presence of an organic solvent. The crude molten bisphenol-A is first dissolved in a suitable organic solvent and the solution is then cooled to yield pure bisphenol-A crystals, which are recovered by filtration. However, this process suffers in that (1) the crystals produced are fine, powdery and needle-like and are difficult to handle, store and dry; (2) these crystals permit some impurities and organic solvent to occlude during crystallization, these impurities cannot be removed during the drying step and hence they are present in the polymerization process and (3) these processes require the use of organic solvent.
A method which uses water as a crystallization medium for crude bisphenol-A is described in U.S. Pat. Nos. 3,326,986 and 3,277,183. According to the disclosure in U.S. Pat. No. 3,326,986, the isolated crude bisphenol-A in molten form is purified by first mixing with water and cooling the mixture to yield large, rhombic crystals of bisphenol-A. The crystallization in water does not provide purification; however, separation of these crystals from the mother liquor, followed by an organic solvent wash, results in purified bisphenol-A. Although the process described within the above-referenced patent avoids occlusion by an organic solvent during the crystal formation step and yields large, less needle-like crystals that are easy to handle, the purification obtained is limited and large quantities of organic solvent are required to wash the crystals.
The disclosure in U.S. Pat. No. 3,277,183 teaches that crystallizing crude bisphenol-A in hot water containing sodium hydroxide provides purification of the bisphenol-A. A process described within the above-referenced patent does not require organic solvent to obtain a high degree of purification. However, the crystals obtained are needle-like and difficult to handle and permit occlusion of impurities. Therefore, the purification obtained by this process is limited. In addition, high temperatures are utilized in the presence of caustic, which is undesirable since bisphenol-A is subject to increased degradation or cracking at high temperatures.
The process described in Ser. No. 443,344, referenced above, describes a washing procedure for purifying aqueous crystallized bisphenol-A with a water/organic solvent wash. Although the process provides large, purified crystals of bisphenol-A which are easy to handle, the process comprising this invention provides crystals of high purity and is much simpler to execute due to the absence of organic solvent.
The process comprising this invention provides a method of obtaining large bisphenol-A crystals of high purity without utilizing an organic solvent and with less degradation of bisphenol-A product than the process described in U.S. Pat. No. 3,277,183.